The present invention relates to a gas collecting unit suitable for the examination of a patient's respiratory gas and supplied from one or a plurality of gas sources with a gas to be respired by a patient, and which gas collecting unit is used for delivering a gas, which at least partially contains a gas expired by a patient, to a measuring device for examination, said gas collecting unit comprising a dome having a shell which at least partially encloses a gas space and which dome includes an opening for pushing the head of a patient therethrough at least partially into the gas space within the dome, and a collar having a wall which extends around the dome opening and which collar includes a wall-surrounded neck opening for introducing the head of a patient therethrough towards the gas space and the wall of said collar connecting the dome shell against the skin of a patient, and the gas collecting unit consisting of said collar and dome separating the surrounding gas space from that remaining inside.
In a variety of medical measurement and treatment procedures, the gas respired or intended to be respired by a patient must be isolated from the ambient air. A reason may be, for example, that a gas received by a patient for respiration must have a composition that differs from ambient air in a desired fashion. Another reason may be the need of measuring the concentrations of a gas respired by a patient, in which case the mixing of ambient air with the respiratory gas would produce a measuring error. Another possible reason is that the gas respired or intended to be respired by a patient could, upon seeping into the ambient air, produce a hazard to others in the room or extra costs as a result of wasting expensive gas.
There are several ways of preventing the mixing of a gas respired by a patient and the ambient air. One possibility is the isolation of a patient in a space totally sealed from ambient air and containing a gas with a controllable composition. Another possibility is to use a mask which seals against the face of a patient, covers the mouth and nose and through which the patient breaths.
Drawbacks encountered in the former method include e.g. the large size of a required apparatus and the resulting expensive price and complicated operation. Also, various procedures on a patient may become impossible or more difficult to perform with a patient in a totally isolated space. On the other hand, the latter method is not suitable for long-term use since, in order to retain its tightness, a mask requires continuous pressing against the face, which might cause blood circulation problems or even damage to the face. It is also true that a conscious patient may find a mask unpleasant and injuries possibly existing in the face can prevent the use of a mask.
A third method can be considered as an intermediary between those described above with just part of a patient, generally the head, being isolated from ambient air. This is usually done by closing the head of a patient in a gastight chamber. One example of such a case is a gas collecting unit for a metabolic measuring apparatus described in U.S. Pat. No. 4,763,664. A gas collecting unit is often referred to as a canopy. A problem in this type of equipment may be the sealing against the skin of a patient at the opening used for passing the head or upper body of a patient into the canopy. This opening need not be completely sealed provided that the examination or treatment procedure in question allows for a leak in either direction. However, there are such methods existing and in use, wherein a leak is not acceptable or it should be as negligible as possible.
In several devices, wherein it has been considered necessary to enclose the head of a patient within a gastight gas collecting unit, the sealing against the neck is effected by compression. Thus, the gas collecting unit is provided with a neck opening which is larger than the neck in its inoperative condition and upon sealing it is compressed to a smaller diameter and as tightly as possible against the neck. A .result of this is that the rim of the opening may be left with tightness-affecting creases upon decreasing the circumference of the opening. Also, the required compressive force may be sufficiently strong to interfere with the circulation and even respiration of a patient. Furthermore, the mechanical design of such sealings based on reducing the size of a neck opening can be quite complicated.